[BUGFIX] PHP warnings in ElementBrowser

The ElementBrowser tries to access array indexes
which do not exists.
Fix this to prevent PHP warnings.

Resolves: #54995
Releases: 6.2, 6.1
Change-Id: Ief660ae535f01bc593617667ce7eb81f8cd78cd3
Reviewed-on: https://review.typo3.org/26824
Reviewed-by: Oliver Klee
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind